As always with minor updates, this suspiciously looks like this version patches some critical vulnerability of the PS4 System. In Sony’s words:
This system software update improves the quality of the system performance.
There’s no confirmation at the moment if firmware 4.05 patches any vulnerability, and there is not enough confirmation for now if the Jailbreak announced yesterday for 4.01 is real, and to what level it is related to this 4.05 PS4 System Update. The people behind the alleged 4.01 Jailbreak, however, have stated that they will disclose their exploit to Sony. This could have already happened and would be an explanation for this new firmware update.
You can expect PS4portal Jailbreak for 4.05 Soon.